(1) Field of the Invention
The present invention generally relates to thermally-processable image recording materials including a di- or triarylmethane thiolactone dye precursor, an organic silver salt, and a binder. More particularly, the present invention is directed toward such image recording materials which further include a particular substituted purine compound.
(2) Description of the Related Art
Thermally-processable image recording materials utilizing di- and triarylmethane thiolactone dye precursors and an organic silver salt are known in the art. Examples of such image recording materials are provided in U.S. Pat. Nos. 4,904,572; 5,196,297; 5,198,406; and 5,278,127; all assigned to the assignee of the present invention. Further examples are provided in European Patent Application 250,558. In thermographic embodiments of such materials, images are typically formed by image-wise heating of the media, whereas in photothermographic embodiments, the image recording materials are initially exposed to light followed by thermal processing. Processing typically involves only heating and does not usually require treatment with water or any other liquids.
After processing, such image recording materials may exhibit a progressive formation of a yellowish-brown color, particularly noticeable after prolonged exposure to light. Although not fully understood, it is believed that this progressive color formation, hereinafter referred to as "gilding", is due to a silver photolysis process causing a bulk-scale reduction of non-photosensitive silver. The gilding process should be distinguished from phenomena such as fog, which is generally attributed to development of unexposed photosensitive silver. Fog and problems associated with instability of photosensitive silver are addressed by the prior art. For example, U.S. Pat. No. 4,170,480 discloses a photothermographic material which includes a 1,2,4-triazole compound for decreasing fog associated with photosensitive silver. Similarly, U.S. Pat. Nos. 3,615,616; 3,801,318 and 3,785,814 describe compounds for reducing fog in photographic materials. More specifically, U.S. Pat. No. 3,615,616 describes a photographic material including a photosensitive silver halide and a bistetrazole compound for decreasing fog. U.S. Pat. Nos. 3,785,814 and 3,801,318 disclose a photographic diffusion transfer material which utilizes a substituted purine as a silver halide complexing agent in combination with a silver halide solvent, for decreasing fog.